The present invention relates in general to pulse oximetry and, in particular, to a pulse oximeter capable of taking multiple digital samples per source cycle and allowing for processing of digital signals for improved consistency and an improved signal to noise ratio.
Pulse oximeters determine an oxygen saturation level of a patient""s blood, or related analyte values, based on transmission/absorption characteristics of light transmitted through or reflected from the patient""s tissue. In particular, pulse oximeters generally include a probe for attaching to a patient""s appendage such as a finger, earlobe or nasal septum. The probe is used to transmit pulsed optical signals of at least two wavelengths, typically red and infrared, through the patient""s appendage. The transmitted signals are received by a detector that provides an analog electrical output signal representative of the received optical signals. By processing the electrical signal and analyzing signal values for each of the wavelengths at different portions of a patient pulse cycle, information can be obtained regarding blood oxygen saturation.
Conventional pulse oximeters generally employ time-division multiplexed (TDM) signals. As noted above, the processing of the electrical signals involves separate consideration of the portions of the signal attributable to each of the sources. Such processing generally also involves consideration of a dark current present when neither source is in an xe2x80x9conxe2x80x9d state. In TDM oximeters, the sources are pulsed at different times separated by dark periods. Because the first source xe2x80x9conxe2x80x9d period, second source xe2x80x9conxe2x80x9d period and dark periods occur at separate times, the associated signal portions can be easily separated for processing.
The algorithms for determining blood oxygen saturation related values are normally implemented in a digital processing unit. Accordingly, one or more analog to digital (A/D) converters are generally interposed between the detector and the digital processing unit. Conventionally, the A/D converter is operative to integrate a signal over a source cycle or dark period and to generate a digital value proportional to the integrated signal. It will be appreciated that this digital value represents an aggregated quantity and does not provide information regarding a signal value at any point on a signal waveform or the shape of the signal waveform within integration periods. The resulting digital signal or signals can be correlated to corresponding intensity values by the digital processing unit, and well known algorithms can then be utilized to obtain the desired blood oxygen saturation values.
It has been recognized that TDM pulse oximeters with integrating A/D converters as described above have a number of limitations. First, integrating A/D converters do not provide a digital signal that reflects the shape of the analog detector signal. Ideally, the detector signal portion corresponding to a single source would be a square wave having a high value corresponding to source xe2x80x9conxe2x80x9d periods and a low value corresponding to xe2x80x9coffxe2x80x9d periods. In reality, the noted detector signal portion generally includes a ramp up phase associated with source powering up and a ramp down phase associated with an intensity drop off associated with powering down. The characteristics of the resulting signal vary from source to source or over the operating life of a particular source. Integrating A/D converters generally accumulate charge or otherwise integrate over a sampling period to provide an aggregated value per signal cycle. Such a value provides, at best, cumulative information over the sampling period and does not yield information regarding the signal shape within a source cycle. Accordingly, processing options are limited and substantially no signal phase information is provided within a source cycle.
TDM signals also entail certain system architecture and processing limitations. In many cases TDM signals are demultiplexed using hardware filters or other hardware demultiplexers to form separate signal channels. Each of the channels then has its own hardware signal processing components such as amplifiers, A/D converters and the like. This separate processing introduces potential sources of error such as channel dependent gains and increases the size, complexity and expense of the hardware design. Processor based demodulation of TDM signals simplifies system hardware architecture, but the system speed and accuracy may be limited, particularly in implementations involving more than two light sources.
The present invention is directed to an oversampling pulse oximeter where the analog electronic signal is instantaneously sampled at one or more times per source cycle so as to enable tracking of the signal shape. The invention thereby allows for more accurate analysis of the detector output, allows for reduced noise processing options, allows for reduction in the number of bits in the A/D converter word length and allows for identification of phase characteristics within a signal cycle as may be desired for enhanced digital processing. The invention also provides an architecture that supports signal multiplexing other than TDM, and supports more than two light channels with reduced additional hardware. Such an architecture reduces potential sources of error associated with separate hardware for processing each channel and reduces the complexity and size of the hardware design.
According to one aspect of the present invention, an oversampling pulse oximeter is provided. The oximeter generally includes at least first and second sources for emitting light having first and second spectral contents, respectively. For example, the sources may include a red LED and an infrared LED. The pulse oximeter further includes a drive system for pulsing each of the sources such that the sources output first and second optical signals, respectively, wherein each of the signals includes a series of high output periods separated by low output periods. The high output periods generally correspond to an xe2x80x9conxe2x80x9d period of the LED and the low output periods correspond to an xe2x80x9coffxe2x80x9d state of the LED. It will be appreciated that the LEDs may output substantial photonic energy even during the xe2x80x9coffxe2x80x9d periods. In addition, the intensity of the LED may vary substantially within an xe2x80x9conxe2x80x9d period. The pulse oximeter further includes: a detector for receiving the optical signals and providing a detector signal having a detector signal waveform representative of the optical signals; a digital sampler for receiving the detector signals and providing a digital output based on the detector signal, and a processor for using the digital signal to perform processing steps related to determining an oxygen saturation related value. The oxygen saturation related value may be a calculated blood oxygen saturation, a value of an oxygen related analyte, or other value related to an oxygen content of the patient""s blood. In accordance with the present invention, the digital sampler is operative for providing a digital value corresponding to a value of the detector signal corresponding to a portion of the detector signal waveform having a time component that is substantially shorter than a cycle of the sources. Preferably, the time component of the detector signal waveform portion measured by the digital sampler is no more than about 10%, and more preferably no more than about 5% of a cycle period of one of the sources. In addition, the digital sampler is preferably operative for providing multiple digital values corresponding to a single high output period and/or a single low output period of one of the first and second sources.
The digital sampler preferably samples the detector signal at a sufficient rate to accurately track the shape of the detector signal and eliminate or otherwise account for potential sources of measurement error. In this regard, the sampler preferably takes at least about three digital values corresponding to a single high output period of one of the first and second sources. More preferably, the digital sampler has a sampling rate sufficient to obtain at least about 10 and even more preferably at least about 20 digital values corresponding to a single high output period of one of the first and second sources. Such a sampling rate allows for accurate detection of a signal phase of interest and also provides for processing involving data averaging which increases the signal to noise ratio of the digital signal under analysis. Such averaging also increases the dynamic range of the system for a given A/D converter word length, allowing use of the converter of a lower word length. In addition, such a high sampling rate can allow for identification of measurements contaminated by electromagnetic radiation by taking a sufficient number of measurements to recognize incorrect variation in the detected signal. Such averaging also improves consistency of the measurements provided by the pulse oximeter.
Optionally, a fast A/D converter may be used in accordance with the present invention to detect a phase of interest of a detector signal. The associated apparatus comprises: an analog to digital converter, operatively disposed between oximeter detector circuitry and a processing unit, for receiving at least a portion of an analog electrical signal from the detector circuitry and converting the received analog electrical signal portion into a digital signal; a phase detector for analyzing the digital signal to detect a phase of interest relative to the digital signal; and processing logic for processing the digital signal based on the detected phase of interest. For example, the detected phase of interest may be used to identify a subset of digital values from the A/D converter corresponding to a center band portion of an xe2x80x9conxe2x80x9d period of one of the sources or other phase of interest. In this manner, variations associated with signal ramping can be accounted for. In addition, the phase detector may be used to synchronize a demodulation function to a modulating signal for a light source, and to trigger a buffering time period to ensure buffering of values corresponding to an integer number of xe2x80x9conxe2x80x9d periods for a light channel.